Most of the previous studies pertaining to the formation of fibrin, the structural basis of blood clots, have been carried out in non-flowing, in vitro systems. Preliminary results show that the presence of flow or hydrodynamic shear forces during the polymerization of fibrin (on the same order of magnitude as those occurring in the vascular system) can have a dramatic effect on the rate of fibrin formation, as well as the size and shape of the formed polymer. In this research, the rate of fibrin polymerization as a function of shear rates will be determined. Also a physical chemical explanation for the effect will be sought in terms of the particle aggregation theory and kinetics of polymerization.